1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-101864, filed on Apr. 9, 2008, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a radio communication system including a base station and a mobile station and, more particularly, to a communication method, a base station, and a mobile station in the system.
2. Description of the Related Art
Currently, various radio transmission schemes, such the third-generation mobile telephone (3G), standard specification for broadband radio access (WiMAX: Worldwide Interoperability for Microwave Access), and standard specification for wireless LAN (WiFi: Wireless Fidelity), have been proposed and are in practical use. However, these different technologies provide different transmission rates and different coverages. Since the transmission rate and coverage are in a tradeoff relationship in general, it is impossible to achieve a high transmission rate and wide-area coverage at the same time with a single radio transmission system. Therefore, there are some cases where an appropriate radio transmission scheme (RAT: radio access technology) is employed according to various circumstances, with the result that cells using different radio transmission schemes are adjacent to each other. For such a radio communication system in which a plurality of radio transmission schemes coexist, several handover techniques are proposed that can be used when a radio terminal capable of operating with both of these radio transmission schemes moves from a cell using one of the radio transmission schemes to a cell using another one of the radio transmission schemes.
For example, in a mobile communication system disclosed in Japanese Patent Application Unexamined Publication No. 2001-54168 (JP2001-54168), a mobile terminal receives radio signals of all communication systems (radio transmission schemes) that the mobile terminal can use, identifies the type of each communication system and also calculates the quality of service (QoS), from the respective signals' frequencies, and then reports these results to a base station (source base station) the mobile terminal is currently communicating with. When the mobile terminal is notified from the network side of a target base station to switch to in response to the report, the mobile terminal secures a radio link with each of the source base station and the target base station and then switches to the target-side radio transmission scheme.
Moreover, according to the Inter RAT handover described in 3GPP TS 36.300 V8.3.0 (2007-12), a target system provides information regarding its RAT (including the radio resource structure, target cell system information, and the like) to a mobile terminal via a currently communicating source system, thus enabling the mobile terminal to switch to the target system's RAT.
However, according to the method in which a mobile station identifies the type of a signal source's radio transmission scheme based on the frequency of a received downlink radio signal, the type of a radio transmission scheme cannot be identified from a frequency in use, in a system, such as a cognitive radio system, where a plurality of radio transmission schemes may use available frequencies.
Moreover, in a case where a plurality of radio access schemes coexist in a single radio communication system such as a wireless LAN (IEEE 802.11), in which IEEE 802.11a/IEEE 802.11g using multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) and IEEE 802.11b using a single carrier coexist, a radio access scheme cannot be identified from a frequency in use, because IEEE 802.11g and IEEE 802.11b use the same frequency.
Another possible method for identifying a radio access scheme is that a mobile station receives a common control signal periodically transmitted by a base station and, based on the result of demodulating the signal, identifies the radio access scheme used by this base station. However, according to this method, since the mobile station does not identify the radio access scheme of a target base station until the mobile station receives a common control signal, a delay of about twice an interval between common control signal transmissions, or longer, may occur, causing a delay in handover processing, for example.